The present invention relates to a positioning device, a positioning control method, a positioning control program, and a computer-readable recording medium having a positioning control program recorded thereon.
A positioning system has been used which locates the present position of a GPS receiver utilizing a satellite navigation system such as a global positioning system (GPS).
The GPS receiver receives a clear and acquisition or coarse and access (C/A) code which is one type of pseudo-random noise code (hereinafter called “PN code”) carried on a radio wave from a GPS satellite (hereinafter called “satellite radio wave”) based on a navigation message indicating the orbit of the GPS satellite and the like (including approximate satellite orbital information: almanac, precise satellite orbital information: ephemeris, and the like). The C/A code is a code which forms the basis for positioning.
The GPS receiver determines the GPS satellite which has transmitted the received C/A code, and calculates the distance (pseudo-range) between the GPS satellite and the GPS receiver based on the transmission time and the reception time of the C/A code, for example. The GPS receiver locates the position of the GPS receiver based on the pseudo-range between the GPS receiver and each of three or more GPS satellites and the position of each GPS satellite in the satellite orbit (see JP-A-10-339772).
The GPS receiver synchronizes the received C/A code with a C/A code replica in the GPS receiver, and calculates a phase indicating the maximum correlation value (hereinafter called “code phase”). The GPS receiver calculates the pseudo-range using the code phase.
Specifically, the C/A code has a bit rate of 1.023 Mbps and a code length of 1023 chips. Therefore, it is considered that the C/A codes line up in units of about 300 kilometers (km) at which a radio wave advances in 1 millisecond (ms). Therefore, the pseudo-range can be calculated by calculating the number of C/A codes existing between the GPS satellite and the GPS receiver from the position of the GPS satellite in the satellite orbit and the approximate position of the GPS receiver, and determining the phase of the C/A code.
Since the C/A code is carried on the satellite radio wave, it is necessary to synchronize the C/A codes and synchronize the carrier frequency (intermediate-frequency (IF) carrier frequency) of the received satellite radio wave with the frequency inside the GPS receiver (hereinafter called “frequency synchronization”) in order to accurately synchronize the C/A codes.
When the correlation result (coherent result) can be output in time units as short as 1 millisecond (ms) due to the high signal strength of the satellite radio wave, the frequencies can be synchronized by forming a phase locked loop (PLL) which corrects the frequency based on the coherent result (see paragraph [0020] of JP-A-2003-98244, for example).
However, the frequencies cannot be synchronized using the PLL when the strength of the satellite radio wave is low. It becomes impossible to synchronize the codes with the lapse of time.
A technology has been proposed which sets an estimated IF carrier frequency by estimating the original IF carrier frequency, and reduces the difference in signal level between a frequency higher than the estimated IF carrier frequency by a specific value and a frequency lower than the estimated IF carrier frequency by the specific value to bring the estimated IF carrier frequency closer to the true IF carrier frequency (see JP-A-2003-255036, for example).
In order to cause the phase of the C/A code replica generated in the GPS receiver to coincide with the phase of the received C/A code, a correlation process is performed while changing the phase of the C/A code replica. Note that the correlation process is performed while changing the reception frequency. Description thereof is omitted from this specification.
A graph indicating the correlation value in coordinates of which the horizontal axis indicates the phase and the vertical axis indicates the correlation value theoretically forms an isosceles triangle having the maximum correlation value as the vertex. A method has been used which generates C/A code replicas with a phase (EARLY) or a phase (LATE) which advances or is delayed by a specific amount from a phase (PUNCTUAL) considered to be an intermediate phase, correlates the C/A code replicas with the phases EARLY and LATE with the received C/A code, and controls the phases of the C/A code replicas so that the correlation values become equal. The intermediate phase between the phases EARLY and LATE when the correlation values of the phases EARLY and LATE are equal is estimated to be the phase of the received C/A code.
The signal from the GPS satellite may reach the GPS receiver as an indirect wave which enters after being reflected by a building or the like (hereinafter called “multipath”) in addition to a direct wave. In this case, the isosceles triangle having the maximum correlation value as the vertex is deformed, whereby the phase of the received C/A code cannot be accurately estimated by the above method.
A technology has been proposed in which the correlation process is performed while reducing the difference between the phases EARLY and LATE (narrow correlator technology) (JP-A-2000-312163, for example).
However, the following two problems occur when the signal strength of the satellite radio wave is extremely low.
A first problem is that it is necessary to appropriately determine the estimated IF carrier frequency. Specifically, the estimated IF carrier frequency cannot be appropriately determined when the signal strength of the satellite radio wave is extremely low.
A second is that, when the signal strength of the satellite radio wave is extremely low, the correlation values of the phases EARLY and LATE become equal at a plurality of positions in the graph indicating the correlation value, as shown in FIG. 26. For example, when the phase EARLY is a phase Qe1 and the phase LATE is a phase Qe2, the correlation values of the phases Qe1 and Qe2 are equal, and the intermediate phase between the phases Qe1 and Qe2 is a phase Qe3. However, the phase Qe3 differs from the true phase Qr.
As described above, when the signal strength is extremely low (electric field is weak), the phase of the received C/A code may not be accurately estimated by the above narrow correlator technology. In this specification, the term “signal strength” is used synonymously with the term “radio wave strength”.